Mississippi’s bruising GOP Senate primary, which voters will decide Tuesday in a runoff (get live results here! ), has come at great cost — more than $17 million — to Republicans.

More than 30 Republican-friendly outside groups, from Club for Growth Action to the U.S. Chamber of Commerce, have lavished in excess of $11.3 million on the race, according to the Center for Responsive Politics. That’s almost twice the $6.1 million spent by Sen. Thad Cochran and his tea party challenger, state Sen. Chris McDaniel.

Liberal groups have essentially sat out the Mississippi primary, rated a Safe Republican contest by the Rothenberg Political Report/Roll Call. As of early May, conservative groups had spent three times more attacking one another in primaries around the country than they had against Democratic candidates, according to an analysis by the Center for Public Integrity.

“This is the tea party’s last stand, and their best chance to knock off an incumbent senator,” said Kevin Broughton, communications director for the Tea Party Patriots Citizens Fund, which has spent close to $800,000 on McDaniel’s behalf. Broughton called the contest “an important race for Mississippians and for the conservative movement as a whole.”

McDaniel has also reaped the rewards of $1.3 million in spending by Senate Conservatives Action, and has been buoyed by a barrage of last-minute ads, rallies and door knocking organized by dozens of conservative groups, small and large. These include a political action committee called Conservative Strikeforce and the California-based Tea Party Express, which weighed in late but is trumpeting its pro-McDaniel bus tour as “ground zero in the battle between the D.C. insider class and the tea party grass roots.”

The biggest spender on McDaniel’s behalf — and in the race as a whole — has been Club for Growth Action, which has shelled out $3.1 million, CRP data show.

For his part, Cochran has gotten a boost from $1.2 million in spending by the U.S. Chamber of Commerce, and another $1.8 million by Mississippi Conservatives, an unrestricted super PAC run by Henry Barbour, the nephew of GOP operative and former Magnolia State Gov. Haley Barbour. The super PAC’s top donors include Sanderson Farms CEO Joe Sanderson, who gave $100,000, and Richard Wax, who owns a company that sells wholesale seeds and bulbs and who gave $50,000.

Four McDaniel enthusiasts face criminal charges after barging in to photograph Cochran’s wife in the nursing home where she lives. The Tea Party Patriots have filed a Federal Election Commission complaint against the Mississippi Conservatives super PAC for receiving a $219,540 loan from Mississippi’s Trustmark Bank. PAC organizers have called the complaint a political stunt.

The race has drawn visits from such nationally known conservatives as tea-party-activist Sarah Palin and former Sen. Rick Santorum, R-Pa. Cochran, in the meantime, has courted African-American voters in what some have cast as a desperate effort to counter McDaniel’s momentum.

Pro-McDaniel forces gained steam after economic professor Dave Brat’s surprise win over House Majority Leader Eric Cantor in Virginia. In an echo of Brat’s attacks on Cantor’s K Street and Wall Street ties, McDaniel’s allies have turned one of Cochran’s central campaign themes — his ability to deliver federal dollars to Mississippi — into a strike against him.

The National Republican Senatorial Committee and Cochran’s Senate colleagues have rallied to his defense, and the incumbent may yet eke out a victory. But as GOP Senate Minority Leader Mitch McConnell and his GOP colleague, Sen. Lindsey Graham, learned in Kentucky and South Carolina respectively, winning doesn’t come cheap.

McConnell trounced tea-party-challenger Matt Bevin in the Kentucky primary, but he spent $11 million in the process. Graham spent $8.5 million to fend off a bevy of GOP challengers in his primary. But that adds up to almost as much as he spent for his entire re-election campaign in 2008, which was $9.7 million.

In conservative Mississippi, the drawn-out GOP primary and runoff have left Democrat Travis Childers, a former congressman, sitting far off on the sidelines. After entering the race in late February, Childers had raised $124,000 by the end of May, according to his April quarterly, pre-primary and 48-hour reports filed to the FEC.

Allies for both GOP candidates have voiced confidence that whoever wins the runoff will go on to win the general election.